7/28/2019 09.1_v.0410_Seismic_CPT

1/2DS9.1v.0410

SeismicCone

Penetration

Testing

(SCPT)

has

proven

to

be

avery

valuablegeotechnicaltoolinfacilitatingthedeterminationof

lowstraininsitucompression(P)andshear(S)wavevelocities.

ThePwaveandSwavesvelocitiesaredirectlyrelatedtothesoil

elasticconstantsofPoissonsratio,shearmodulus,bulkmodulus

andYoungsmodulus.TheaccuratedeterminationofPwaveand

Swavearrivaltimesfromtherecordedtimeseriesisof

paramountimportancetotheevaluationofreliableseismic

wavevelocities.

TheseismicCPTacquisitionisconductedwithaseismicadapter,

attachedtoa10cm2or15cm

2subtractionorcompressionCPT(U)

cone.Theadapterisdesignedsuchthattheseismicsensorsre

cordthesoilprofilesresponsetolowstrainseismicdistur

bances.

InSCPTtestingtherearetypicallytwotypesofsensorsused,

velocity(i.e.geophones)andacceleration(i.e.accelerometers).

Aseismicsensorcanbesatisfactorily modelledasalinearsecondordersystem,withacharacteristic

naturalfrequencyanddampingfactor,whichtransformsthesensorinputofbodywavesandassociated

randomnoisepropagatingthroughtheearthintoanelectricalsignal.

Foragooddefinitionofthefrequencyspectrumoftherecordedsignal,i.e.toavoidsmearing,the

GeoMilseismic

adapter

is

equipped

with

accelerometers

(1

Hz

10

kHz)

and

not

with

geophones

(typically10 300Hz).

Tosimplifythedataacquisition,allhardwareandsoftware

componentsaredesignedtorunwithanotebookcomputer

connectedtoa"SignalConditioningSystem",ratherthanthe

traditionalseismograph.Forqualityassessmentandproperdata

analysis,thefrequencyfilteringofdataisdoneafteracquisition

andnotduringtherecording.Themeasuredvaluesare

transferredasvoltagesignalsbymeansofacableconnectedon

surfacetothe"SignalConditioningSystem"whereCPTdatais

separatedfromtheseismicdata.

TheCPT

data

are

classically

logged

by

the

GME

acquisition

unit

and further processed by the acquisition software CPTest

running on a computer with a Microsoft Windows 32bitoperatingsystem.

ThesignalconditionedseismicdataisA/DconvertedbyaPCMCIAcardandthenprocessedbySC(115)DACsoftware

fromBaziwConsultingEngineers(BCE).TheSCPTsystemscanbeexpandedupto15channels(30channelsiftwoA/D

boardsareused),buttypicallyuniaxial(SC1DAC),biaxial(SC2DAC),triaxial(SC3DAA),andtrue intervalsixchannel

systems(SC6DAA)areused.

Ingeneralterms,theseismicconeisadvancedtothedepthofinterestusingahydraulicreactionarypushingforcelike

generatedwiththeGeoMilstaticpenetrometers.

SeismicCPT

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DS9.1v.0410

AnimportantfactorinSCPTistogenerateclean,strongand

reproduciblesourcewavelets.TheparticlemotionofthePwave

isinthedirectionoftheraypathwhiletheparticlemotionofthe

Swaveisperpendiculartotheraypath.TheSwavecanbe

polarisedbothparallel(SH)andperpendicular(SV)totheground

surface.WhenaPorSVwavestrikesaboundarybothSVandP

wavesaregenerated.ASHwavewillonlygenerateSHwavesat

boundariesandthereforesimplifiesshearwaveseismicanalysis.

SeismicsourcesaredesignedtogenerateeitherdominantP and

SVwaves(e.g.Buffalogunfiredintheground)ordominantSH

waves(e.g.hammerbeam).Thehammerbeamcomprisesof

applyingahammerblowlaterallytothesidesofspecialdesigned

platesfixedtothepenetrometer.Thehammerbeamgenerates

excellentpolarisedSHwaveletsanditisstandardappliedin

reversepolarityanalysis(Baziw,1988).

BCEprovides

both

the

SC(1

15)

DAC

data

acquisition

and

the

SC(1

15)

RAV

data

reduction,

analysis

and

visualisationsoftwarepackages.SC(115)RAVimplementspatented(i.e.,USPatent#5,177,709and

CanadianPatent#2,077,387)DSPsoftwareinderivingseismicvelocitiesandattenuationQvaluesfrom

SCPTdata.

Fromtherecordedtimeseries,arrivaltimesfortheSwavesandPwavesaredeterminedand

correspondingvelocityprofilesderived.Theaccuratearrivaltimeestimatesarecriticalsothatthe

investigatorisconfidentinthederivedvelocityestimates.

PwaveandSwaveincrementalvelocitydeterminationis

directlydependentuponthequalityoftherecordedseismictime

seriesandtheabilitytoextractthedesiredwavelets(i.e.,PorS)

understudy.TheseismicPandSbodywavesandambient

backgroundand

electrical

noise

constitute

the

recorded

seismic

timeseries.Theabilitytospectrallyidentifyanddigitallyisolate

thedesiredwaveletsrequiresfastresponseandhighbandwidth

sensors.Slowresponseandlowbandwidthseismicsensorsmay

resultinunrecoverabledistortionsintherecordedtimeseries.

AccurateinsituPwaveandSwavevelocityprofilesareessential

ingeotechnicalfoundationdesigns.Theseparametersareused

inbothStaticandDynamicSoilAnalysiswheretheelastic

constantsareinputvariablesintothemodelsdefiningthe

differentstatesofdeformationssuchaselastic,elastoplastic,

andfailure(Finn,1984).

Anotherimportantuseofestimatedshearwavevelocitiesingeotechnicaldesignisintheliquefactionassessmentofsoils.Sincetheshearwavevelocityisinfluencedbymanyofthevariablesthatinfluence

liquefaction(ge.,voidratio,soildensity,confiningstress,stresshistory,andgeologicage),itisan

excellentindexofliquefaction.

TheseismicconehasproventobeaveryaccurateandreliabletoolinthedeterminationofVsandVp

profiles.Themanyadvantagesoftheseismicconeconsistsofexcellentsoilprobecoupling,acontrollable

source,andcosteffectivenessbecauseitisaretrievableprobe.Detailsoftheseismiccone,thedownhole

testprocedures,andcomparisonswiththecrossholeresultsatseveralsiteshavebeendescribedby

Campanellaetal(1986).